The sad part about all of this is that you could make a case for the invasion being the best option for China during this time. The 20th Century was a time of disaster after disaster for the Chinese people, but the worst of these were the ones inflicted by the CCP on its own people. Even with the use of nuclear weapons on major Chinese cities and invasion and battle deaths, I doubt you'd see deaths much higher than a few million. Even with the Clean Fields/Three Alls policy, the IJA didn't manage to cause more than about 6 million deaths in China, and it wasn't from lack of effort. Compare this to the figures for the Great Leap Forward, which is estimated to have taken between 20 and 44 million lives. Almost any war will end up saving Chinese lives if it ends with Mao's fall from power. Of course, we only know this with the benefit of hindsight. I don't doubt that, in the event of a US invasion bringing a new government to China, the Chinese people would not think the US had done them any favors.
It is all academic in any case--I agree with the others that said the US (and whatever UN forces could be talked into coming along, which I doubt would be too many) would not be able to conquer China. Even with nukes, even with the US putting the same kind of effort into this war that was put into WWII, I think the US would run into the same problem that the Japanese Army did. It would be able to destroy any opposing army that it met in the field, but it would control only the land directly under the boots of the soldiers. As soon as the soldiers moved on, the people will go back to helping repel the invaders. China can always trade space for time.